The present invention relates to a non-volatile memory device, and more specifically to an electrically erasable programmable read-only memory (EEPROM).
A conventional non-volatile memory includes in general, a timing controller, a step-up circuit, an address buffer, a row decoder, a column decoder, a read/write controller, a memory cell array composed of a plurality of transistors each having a floating gate and arranged into an array, and an I/O buffer.
In the conventional non-volatile memory, there exists a limitation in the number of operations or activations that data can be written into each memory cell of the memory cell array (referred to as endurance, hereinafter). This is because the insulating film and the floating gate of each memory cell transistor deteriorate, due to the tunnel effect, such that electrons are injected into or emitted from the floating gate of the transistor through the insulating film. Therefore, as shown in FIG. 8, a difference in threshold voltage between a curve g.sub.1 representative of the logical "1" side and a curve g.sub.2 representative of the logical "0" side decreases with increasing endurance, so that the threshold voltage margins h.sub.1 and h.sub.0 on both sides are reduced, respectively. As a result, it has been necessary to prevent erroneous data reading operation by previously determining a limitation of the endurance (e.g. 10.sup.4 times).
Accordingly, in the conventional non-volatile memory, care must be taken so that the number of operations that data can be written will not exceed a predetermined limit or a guaranteed range. Especially, in such a system configuration that data are often rewritten at one part of the memory cells, even if the other part of the memory cells lies within the guaranteed range or if erroneous operation does not at all occur in practice, whenever the memory cells at which data are concentratively often rewritten reach the number of predetermined guaranteed operations, it has been necessary to replace the old non-volatile memory with a new one, or to change data writing operation via an external controller, so that data may be concentrically rewritten to different memory cells.